1. Field of the Invention
The present invention relates to a hydraulic control device particularly designed for an automatic transmission of an automobile having a multiple speed transmission such as a five speed automatic transmission, and in detail relates to a hydraulic control device for setting gear ratios which are different from those of a conventional automatic drive range during a "manual-shift-down" such as from Drive to 3rd, 2nd and 1st range by a shift lever.
2. Description of the Prior Art
An automatic transmission, in general, performs a shift down operation through an ordinary gear ratio as well as manual shift down performed by a shift lever to reduce the speed of a vehicle on a long down slope and when passing other cars.
An applicant of the present invention, as shown in the Japanese Pat. No. 57-37140, proposed an automatic transmission having a sub-transmission mechanism (two forward speeds) and a main-transmission mechanism (three forward speeds), with the transmission of both sub and main transmission mechanisms operated by respective shift valves, which makes possible an automatic transmission having six forward speeds. The thus proposed automatic transmission has six forward speeds in the Drive range from the combination of sub and main transmission mechanisms, while on the other hand, the automatic transmission has three forward speeds in the combined intermediate and low ranges when the submechanism is not used.
Accordingly, a conventional three or four forward speed automatic transmission only regulates shifting to a higher speed with an ordinary gear ratio and cannot select a suitable gear ratio for the running condition even though manual shift down is performed to obtain higher acceleration when passing other vehicles or applying the engine as a brake when a vehicle is running on a long down slope.
Furthermore, a multiple speed automatic transmission device having a main transmission mechanism and a sub transmission mechanism can have different shifting patterns in the Drive range and the intermediate and low ranges; however, in such an arrangement, three solenoid valves are required for first, second and third shift valves respectively. Incorporating expensive solenoid valves in the system causes the total cost of the automatic transmission to increase.